Holley Truck Avenger tuning

Eric Sj.

Member
Joined
Mar 15, 2018
Location
Hillsborough
Hey guys and thanks in advance for any knowledge shared here. As the title says I've got a couple bugs to work out of my carb and am looking for help from those who have done it before.

Here's my set up:
Holley Truck Avenger 470 (electric choke) on a 1981 CJ5 with the 258. I have an Offy intake and a set of headers on it, other than that the engine is stock. Current power valve is a 6.5, primary jets are 57's and secondaries are 61's. Vaccume at idle is about 21

Here's my problem:
The engine idles at anywhere from 1200 to 1600 rpms, BUT occasionally will idle at 800 or so (maybe 1 in 5 drives). The throttle cable is not sticking, the idle adjustment screw is turned all the way out and the choke does open all the way. Am I running too lean?

Other than the idle this set up is running fantastically. Throttle response is quick, power is constant and the engine only bogs ever so slightly when shifting into third gear.


What am I missing!?
 
I know you said the throttle cable isn't sticking but have you checked to see if the throttle blades are actually fully closed with the adjustment backed off. Easiest way to tell is grab the throttle on the carb and push is closed. If your idle comes down then you need to figure out why it's not returning closed. Could also be a choke issue as when the choke is engaged the throttle will not be able to close all the way.
 
Used carb. I've cleaned it pretty well. may have a sticking needle seat in the primaries, could that do it?

I have checked the throttle blades and again no luck.
 
So the idle speed adjustment screw is all the way out. If the throttle blades are actually closed all the way in the bores, then you are leaking air in somewhere. Plug all your vacuum ports and check for vacuum leaks.
 
I had a similar issue with the 670 truck avenger on my buddy's truck. It was a rubber vacuum cap on the back side of the base plate...it had dry rotted and cracked after a few months. Cheap rubber. Start simple!
 
Okay, if the idle speed screw is backed all the way off and it's still idling at 1600, you are getting air in there somehow.

As mentioned before, first check and make sure your cable/bracket setup isn't keeping the primaries open a bit. If not, you're getting air from somewhere. Gotta hunt that down first, because working on anything else is irrelevant if it's sucking air from somewhere that's supposed to be sealed.
 
I'll 2nd (3rd? 4th? eh...Nth) with the vacuum leak plausibility. You're definitely getting air in from somewhere. Is the return spring functioning properly? If you try and close the throttle linkage by hand, does that bring the idle speed down?

Another area of concern is the base plate/manifold gasket. Spray some carb cleaner around the base while it's idling. If it changes the idle speed at all, there's an issue. It can be also possible (hopefully not for your case) that the base plate is warped. This could also be from inconsistent torque on the nuts holding it to the manifold.

But again, I'd start simple and hunt down some vacuum leaks...rotted caps, cracked vacuum hoses, etc. Could be that your vacuum advance hose is cracked.

Which spurs a next thought - where did you connect your vacuum advance? Some people swear by connecting it to a manifold vacuum port. I personally prefer having the vacuum advance connected to a ported/timed passage.

Eric Sj. said:
Used carb. I've cleaned it pretty well. may have a sticking needle seat in the primaries, could that do it?

I have checked the throttle blades and again no luck.
Click to expand...

If you had a stuck needle, chances are that it would stick open/not seat fully, and you'd be running incredibly rich or have raw fuel coming from the exhaust. Another tell-tale from this is a fountain of fuel coming from the bowl vent.
 
Sorry for the slow response guys,

First: I've been all over this thing with carb cleaner while running (base plate, intake, vacuum ports) and haven't found a single spot that is leaking.

Second: I had a buddy come by and tinker with the carb for a little and he adjusted the "fast idle adjustment screw". The Jeep has gone from idling at 1600 and 1100rpms to 1050 all the time.
I need to clarify that before adjustment it would idle both hot and cold at somewhere between 1100 and 1600 rpm, depending on what it decided it wanted to do. I could drive for 20 minutes ont he highway just to hit a stoplight, hold the clutch in, and have an idle of 1400. It was completely random, other times it would drop back down to 1100 or so which was much more bearable.

1050 is a huge improvement but I'd really like to have my nice low 750 idle back. Any more ideas? Any explanation as to why the "fast idle screw" made a difference?


Thanks to all!
 
The fast idle screw engages a cam that is engaged by your choke. Basically, when the engine is cold, and you pop the throttle a couple times before you crank it over, the thermostatic spring in the choke will close the choke flap and advance the fast idle cam. It sounds to me like the screw is backed off some, but still touching the cam. Once the engine is warm, you don't need the screw touching the fast idle cam at all. If it is like some newer carbs, the cam will have several steps in it anyway. I'd back that screw off more until you find the idle in a happy place for you. I was going to say "not touching the cam when the engine is warm," but after looking at the fast idle setup on the truck avenger, it seems to be different than some other 4150 Holleys. Visual aid shown here:

fast-idle-screw-jpg.70160


I'd back it off some more. What does the engine idle at cold? I'd have to assume pretty high. At least back it off until your regular idle screw is actually functioning for you again.
 
Honestly, around these parts you shouldn't really need a choke. Take that bastard off and see if your problem goes away.
 
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